The ends of eukaryotic chromosomes, called telomeres, perform several vital functions. One function is self-maintenance; because loss of DNA from the terminus is inherent in replication, this loss must be counterbalanced by addition of telomeric DNA to the terminus. The long- term goal of the research proposed here is to understand the structure of Drosophila telomeres, how they are maintained, and how they function. It appears that Drosophila telomeres are maintained by a novel mechanism involving the occasional transposition to the terminus of specialized families of non-LTR retrotransposons. Non-LTR retrotransposons, also called LINEs, are a class of mobile elements which is ubiquitous among metazoans and has been implicated in causing mutations in man. Two families of telomere-associated LINEs, HeT-A and TART, have been found in Drosophila. A long-term objective of this project is to determine the basis for the unusual transpositional specificity of these elements. One specific aim proposed in this application is to show that HeT-A and TART elements are at the termini of Drosophila chromosomes by identifying terminal DNA fragments hybridizing with DNA probes for these elements. A second aim is to survey the number, DNA sequence organization, and chromosomal locations of TART elements in D. melanogaster and other Drosophila species. This will be accomplished by genomic Southern blot analysis, in situ hybridization, and DNA cloning. A third aim is to characterize the expression of TART in D. melanogaster. The transcription unit and promoter will be mapped. The tissues and developmental stages in which the transcription and translation products of TART accumulate will also be determined. The fourth aim is to reintroduce a cloned TART element into the germline of D. melanogaster and show that it will transpose to a telomere. This would provide an in vivo transposition assay for future in vitro mutagenesis experiments.